Technical Field
This invention relates to methods and apparatuses that enable a facility or entity that transmits and receives radar signals over a broad frequency range, spanning at least three octaves to receive any incoming signals, while blocking reception of any signals generated by the facility or entity itself.
Background Art
Many facilities or entities, such as ships, airplanes and land based sites need to transmit and receive signals, such as radar signals, simultaneously. This can present serious difficulties since the receiving antennas will often pick up very large signals from the transmitting antennas owing to their close proximity. These signals will mask the incoming signals as described below.
The transmitted and received signals may be continuous, rf signals (CW), or pulsed. The rf frequencies are usually in the range of 500 Mhz to 30 GHz, and the pulse durations are long compared to a single rf cycle. The rf is generally acting as a carrier for the pulses.
The signals detected by the receiving antennas are taken through cables from the antennas to detectors and may pass through filters, rf amplifiers, limiters, isolators impedance matching networks, and the like. A typical setup is shown in FIG. 1(a). The detectors are usually tunnel, or Schottky, diodes with an inductor to ground on the input side and a capacitor to ground on the output side. The inductor provides a DC path to ground for low frequency components, while the capacitor serves to run rf signals to ground after they have been through the detector. The diodes themselves act as “power law” detectors, producing a DC offset proportional to the rf power input on the capacitor side of the detector for CW rf inputs and a pulse with a pulse height proportional to the rf power for received signals which are pulse modulated rf. The DC signal, or demodulated pulses are then amplified by video amplifiers and the information carried by them processed by subsequent electronic circuitry. The detectors produce an output that is proportional to the input power up to the input power of about −20 dbm. For higher input powers the response is less sensitive, and for powers above about 5 dbm, tunnel diodes behave poorly and can be destroyed very easily. Schottky diodes can take higher powers than tunnel diodes without being destroyed, but their sensitivity is also seriously degraded for high power input signals.
If the receiving system is only intended to detect pulses and CW is also present, either from an external source or from the transmitting antennas, it is possible to strip this CW, while maintaining the ability to see and measure the amplitude of short pulses, by using filtering techniques, or “baseline restorers” in the video circuitry that follows the detector, provided that the incoming power is low enough so the detector's sensitivity is not degraded, that is, for rf powers below about −20 dbm. For higher powers it may be still be possible to see and measure the amplitude of pulses accurately provided the power of the rf carrier for the pulses is larger than the power of the CW that is being stripped. However, the loss of sensitivity of the detectors makes it impossible to see small incoming pulses that could be seen in the absence of the CW. In addition, for high input powers of CW, inner modulation noise becomes a serious problem and this further masks the small input signals. The inner modulation noise results from the non linear nature of the detector. Because its output is proportional to the square of the amplitude, if two frequencies are present in the rf, the output of the detector will have one term with a frequency equal to the sum of the frequencies, and another with the difference of the frequencies. Since there is always white rf noise, the difference frequency between the large signal rf received and that of some of the noise will fall directly into the video band, giving rise to the inner modulation noise. Finally, it is not possible to detect and measure pulse amplitudes accurately if the carrier frequency is close to that of the unwanted incoming CW, its subharmonics or harmonics, because of interference effects in the rf. In spite of these problems all of the efforts we are aware of have attempted to strip CW in the video section, after the signal has been detected, or have used many channels, each one with its own detector with channels having narrow filters. With this latter approach all channels except the channel that receives and passes the transmitted signal are live to incoming radar. This is expensive and does not permit reception in the same band that is being used to transmit. Note that incoming signals or CW from distant sources are usually relatively weak so that the detectors are not crippled by these signals. The high power signals that are generally the problem are those generated by the facility's, or entity's own transmitting antennas.
The only way to prevent these problems is to stop the unwanted signal from reaching the detector and a method and apparatus for achieving this is the object and advantage of the present invention.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.